The retina consists of two different types of light sensitive receptors, cones and rods. Cones are responsible for colour vision, while rods help us see in dim light.

The sensitivity of the receptors is not uniform of the entire range of visible wavelengths. Perception begins at 380 nm (blue) and ends at 750 nm (red) with maximum sensitivity at 555 nm (green). This is shown in the sensitivity curve V (l) � photopic vision.
 
Because of the different receptors the sensitivity curve is shifted slightly for night vision and is known as the V' (l) curve - scotopic vision.